This invention relates generally to rechargeable batteries and more particularly to a rechargeable lithium battery capable of discharging to zero volts without causing damage to the battery.
Rechargeable lithium batteries are widely discussed in the literature and are readily commercially available. They typically consist of a positive electrode and a negative electrode spaced by a separator, an electrolyte, a case, and feedthrough pins respectively connected to the electrodes and extending externally of the case. Each electrode is typically formed of a metal substrate that is coated with a mixture of an active material, a binder, and a solvent. In a typical battery design, the electrodes comprise sheets which are rolled together, separated by separator sheets, and then placed in a prismatic case. Positive and/or negative feed through pins (i.e., terminals) are then connected to the respective electrodes and the case is sealed.
The negative electrode is typically formed of a copper substrate carrying graphite as the active material. The positive electrode is typically formed of an aluminum substrate carrying lithium cobalt dioxide as the active material. The electrolyte is most commonly a 1.1 mixture of EC:DEC in a 1.0 M salt of LiPF6. The separator is frequently a micro porous membrane made of a polyolefin, such as a combination of polyethylene and/or polypropylene which can, for example, be approximately 25 xcexcm thick.
It is typical to use protection circuitry with lithium ion batteries to avoid potential deleterious effects. Thus, protection circuitry is frequently employed to terminate charging if the voltage or temperature of the battery(or any cell) exceeds a certain level. Moreover, it is common to incorporate a low voltage cutoff to disconnect the battery from its load if the voltage of the battery (or any cell) falls below a certain lower level. This latter precaution is taken to prevent permanent damage to the battery which can occur if a voltage greater than a Damage Potential Threshold (DPT) is applied to one of the electrodes. For example, corrosion or decomposition of the negative electrode substrate can occur if a voltage greater than a Substrate Dissolution Potential (SDP) is applied to the negative electrode.
The present invention is directed to a rechargeable lithium ion battery particularly configured to be able to discharge to a very low voltage, e.g. zero volts, without causing permanent damage to the battery. More particularly, a battery in accordance with the invention is configured to define a Zero Volt Crossing Potential (ZCP) which is lower than the battery""s Damage Potential Threshold (DPT) and more specifically its Substrate Dissolution Potential (SDP), to thus avoid low voltage substrate damage.
The ZCP refers to the voltage of each of the electrodes relative to a lithium reference (Li/Li+) when the battery potential, i.e., the potential between the electrodes, is zero. The SDP refers to the dissolution potential of the negative electrode substrate relative to the lithium reference (Li/Li+). A conventional lithium ion battery typically exhibits a ZCP of about 3.6 volts which can slightly exceed the battery""s SDP.
In accordance with the present invention, the material selected for the negative electrode substrate has a dissolution potential greater than the ZCP. Commercially pure titanium and titanium alloys are preferred. Nickel, nickel alloys, and stainless steel can also be used.
In the normal operation of a lithium ion battery, a solid electrolyte interface (SEI) layer, i.e., a passivation layer, is formed on the negative electrode, attributable to a reaction between the negative electrode and the electrolyte. The SEI layer comprises an insulating membrane that tends to inhibit the continuing reaction of the negative electrode and electrolyte. It has been recognized that this SEI layer can dissolve at a voltage above a certain level, i.e., Film Dissolution Potential (FDP), which can lead to permanent damage to the negative electrode. In accordance with a preferred embodiment of the invention, the battery is configured to assure a ZCP lower than said FDP.
A battery""s ZCP level relative to the lithium reference is dependent in part on the materials used for the positive and/or negative electrodes. In accordance with a preferred embodiment of the invention, a positive electrode active material, e.g., LiNiXCo1-XO2 (0 less than xxe2x89xa61) is selected which exhibits a discharge curve appropriate to achieve a relatively low ZCP level. This feature of the preferred embodiment facilitates the implementation of a battery in accordance with the invention characterized by a Zero Crossing Potential (ZCP) less than its Substrate Dissolution Potential (SDP) and/or its Film Dissolution Potential (FDP).
Batteries in accordance with the present invention are particularly suited for use in critical applications where physical access to the battery may be difficult. For example, batteries in accordance with the invention find application in medical devices configured to be implanted under the skin in a patient""s body. Such a medical device is typically comprised of a hermetically sealed housing formed of biocompatible material and dimensioned sufficiently small as to be able to be implanted without interfering with normal bodily function. A battery in accordance with the invention includes a case configured for mounting in the device housing. The battery case can be of a variety of shapes, e.g., prismatic or cylindrical, and typically defines a volume of between 0.05 cc and 30 cc. Batteries within this range exhibit capacities between 1.0 milliamp hours and 3 amp hours. An exemplary battery for use in such a device includes a prismatic hermetically sealed battery casing having dimensions of 35 mmxc3x9717 mmxc3x975.5 mm. The device is intended to be implanted in the lower back region to help alleviate back pain using neurostimulation techniques.